The AC/heater (HVAC) system may unexpectedly switch to the defrost mode when accelerating. This system is operated by engine vacuum and should be inspected for any vacuum leaks if this problem develops. There is also a revised vacuum check valve available to address this concern.

External oil leaks from the valve cover gaskets, intake manifold gaskets (front or rear), and the rear crankshaft seal area are common. The rear main seal is an unlikely source. Normally, leaks in this area are from the bearing cap mating surfaces as well as the sealing surface between the oil pan and bearing cap.

Leaks at the front and/or rear differential pinion seal and from the transfer case are common. Leakage from the transfer case normally occurs at the case-mating surfaces and require removal of the transfer case to reseal.

Loose or worn suspension and steering components can cause a wobble or shaking in the steering wheel. It may happen at a certain speed and subside as the vehicle accelerates through the range. Tire balance can contribute to this as well.

Due to warped front brake rotors or variations in the thickness of the rotors, brake pulsations may develop. The best method to reduce pulsations is to use an "On the Car" brake lathe (as long as the rotors are suitable for turning). "On the Car" equipment can correct variations (or "runout") in the rotor surfaces because it also corrects variations in other components (e.g. the hub).

A chirp type noise may be noted from the serpentine belt when the steering wheel is at full lock with the engine idling. Our technicians tell us a revised serpentine belt tensioner is available to correct this concern.

I have a '98 JGC Laredo (4.0 liter, inline-6, 4x4) and it will not shift into gear on it's own. Also it won't go into overdrive. I have repalced the PCM and the shifting silinoid. Any suggestions? Also, the AC compressor just stopped kicking on. what's wrong with it?

One the 3.9L V6 and 5.9L V8 the carbon buildup on the top of the piston is common. As the buildup increases with mileage and over time, symptoms may vary from light ticking, to ticking/hammering, to hammering/knocking noises. Fuel injector cleaner often solves the problem.